Traditionally, an image forming apparatus of a so-called tandem type has been known, in which photosensitive members corresponding to respective colors of yellow, magenta, cyan, and black are arranged in parallel. With the image forming apparatus of the tandem type, toner images for the respective colors are almost simultaneously formed on the respective photosensitive members. Then, with an image forming apparatus of a direct transfer type, toner images for the respective colors are transferred onto a sheet in an overlapping manner, thereby forming a color image on the sheet. On the other hand, with an image forming apparatus of an intermediate transfer type, toner images for the respective colors are transferred onto an intermediate transfer belt in an overlapping manner, so that a color toner image is formed thereon. Then, the color toner image is transferred onto a sheet, thereby forming a color image on the sheet.
As an image forming apparatus of such tandem type, for example, a configuration in which photosensitive drums for respective colors are integrally held in a photosensitive drum unit that is detachably attachable to a casing has been proposed.
However, in the image forming apparatus configured according to the aforementioned proposal, the photosensitive drum unit may be attached out of a normal attaching position in some cases. Such inaccurate attachment will cause displacement of a color image formed on a sheet, or deformation of a color image. As a result, a good image formation cannot be achieved.